Field of the Invention
The present invention relates to an effective and efficient spray drying process for the production of CL-20/HMX cocrystals which results in a desirable nanoscale size range.
Related Art
Cocrystals are unique crystalline structures which consist of at least two different component materials (“co-formers”) in a fixed ratio. While the individual components typically exist as discrete crystalline materials, under suitable conditions compatible crystalline materials may crystallize into a new cocrystalline material, in which the hybrid cocrystalline material contains the coformers in a fixed ratio. Such cocrystalline materials are currently sought after as a means to engineer materials with new properties. Relatively recently, a significant number of new cocrystals have been reported, particularly in the pharmaceutical industry (see, N. Qiao et al., “Pharmaceutical cocrystals: An overview,” International Journal of Pharmaceutics 419 (2011) 1-11).
2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20 or HNIW), is a high density energetic developed by the U.S. military. This high explosive compound has a high detonation velocity and pressure (and thus is a strong explosive). An important drawback of CL-20 is its relatively high shock sensitivity, making it unsuitable for some applications and U.S. Military specifications.
In U.S. published patent application 2012/0305150, to Matzger et al., various pure cocrystalline explosive materials are disclosed, including one containing both CL-20 and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) in a 2:1 molar ratio. And, it is further disclosed that this hybrid explosive cocrystalline material can be effectively used as an energetic filler or propellant component in weapons systems to provide increased anti-armor penetration, enhanced missile payload velocity and flight, increased underwater torpedo effectiveness and lethality, improved gun propellant impetus, and mining and blast applications. In fact, it is disclosed that the CL-20/HMX cocrystalline material has the advantage that it is less impact sensitive, when compared to CL-20, and is a more powerful explosive (i.e., higher detonation velocity etc.) than HMX.
The method of manufacture of the CL-20/HMX cocrystalline material disclosed in the U.S. published patent application 2012/0305150 is evaporative crystallization. More specifically, this evaporative crystallization process disclosed in the 2012/0305150 application involves the cocrystal being formed by evaporating a solution of CL-20 and HMX in any of a number of alternative organic solvents. CL-20 and HMX may be combined in a ratio that promotes the formation of a cocrystal by evaporation. The solution can be sonicated for a short time to aid in dissolution of the CL-20 and the HMX. After sonication, the solution can be decanted and the solids recovered by conventional known means, such as centrifugation; washing, i.e. purifying; dehydration; filtration; or a combination thereof. In various embodiments, a dehydrating agent is added to the slurry to aid in the recovery of the cocrystals. The dehydrating agents include 3A, 4A and 5A molecular sieves. However, it is known that the crystals provided by such evaporative crystallization are substantially pure and relatively large, on the order of about 10 to about 100 microns or greater—and that the resulting explosive materials will not provide the desired lower sensitivity of smaller crystals/particles. See Stepanov et al., “Production and Sensitivity Evaluation of Nanocrystalline RDX-based Explosive Compositions,” Propellants Expos. Pyrotech., 36, 240-246, 2011.
An alternative means of producing CL-20/HMX cocrystals in a nano-sized form, with an average particle/crystal size of 250 nanometers, within a size distribution of from 50 to 400 nanometers, is disclosed in an article by Bing Gao, et al, entitled: Facile, continuous and large-scale synthesis of CL-20/HMX nano co-crystals with high performance spray-assisted electrostatic absorption method. See, Journal of Materials Chemistry A, 2014, Vol. 2, pp. 19969-19974. This article discloses a process wherein the raw CL-20 and HMX are dissolved in acetone at a concentration below the saturation point (to obtain a complete solution). This solution is subjected to ultrasonic atomization, producing fine droplets that are transported by an inert gas to an oven or precipitator, wherein the solvent evaporates and the nano-sized crystals are formed. The fine, particulate product is collected electrostatically. This process, as stated above, results in a product consisting of pure cocrystal particles with an average size of 250 nanometers—and a size distribution of from 50 to 400 nanometers—such that a significant portion of the particles will be from 50 to 250 nanometers. Significant drawbacks of this method include safety issues with such pure, very small particles and difficulties in handling such pure, very small nanoparticles—which present significant challenges in preparation of useful explosive formulations, due in part to difficulties with dispersion and coating of such particles.
Another alternative means of producing the desired CL-20/HMX 2:1 molar ratio explosive cocrystalline material is disclosed in an article by D. Spitzer, et al., entitled: Continuous engineering of nano-cocrystals for medical and energetic applications. See, Science Reports, Vol. 4, p 6575, DOI:10.1038/srep06575. The method of production disclosed by this article involves dissolving the coformer CL-20 and HMX crystals in a low boiling point solvent; using an overpressure of 40 to 60 bars to atomize the solvent into an evacuated atomization chamber by means of a heated hollow cone nozzle. The pressure in the atomization chamber is kept constant at 5 mbar—such that when the atomized solvent is injected there is ultrafast evaporation thereof, which induces crystallization of the solute. The reported resulting cocrystal mean particle size of pure CL-20/HMX 2:1, was 59 nm—and, the subject method reportedly prevents any further growth. Therefore, the pure CL-20/HMX cocrystals obtainable from this method suffer from the same handing and processing difficulties and safety issues as those obtainable from the method disclosed in the article by Bing Gao et al., discussed above.
Considering the above disclosed processes for forming relatively pure CL-20/HMX cocrystals of relatively large and very small crystals, there is a need for method to produce CL-20/HMX cocrystals which do not suffer from the same handling and safety characteristics.